A baby is typically put in a day care because the parents are out having jobs so they can financially support their family - including the baby. If the parents can't work and have to take care of the baby all day, they won't get the money to put a roof over the baby's mouth and food in his/her mouth. Plus, daycares are specifically designed to be able to keep babies.
If a parent wants to pay a sitter for their 4 year old, let them. They are the ones paying for a service and the person providing the service is making their money, so its all good both ways. Don't shame a parent for hiring a sitter for any child over 1 year of age.